Diesel exhaust system including nox-trap

ABSTRACT

An exhaust gas treatment system ( 8 ) for a diesel engine ( 10 ) comprises a NOx absorber ( 28 ) charged with solid absorbent for absorbing NOx from a diesel exhaust gas and means ( 18 ) for introducing intermittently into the exhaust system CO upstream of said absorber ( 28 ) whereby the solid absorbent is regenerable by contact with the CO-enriched gas. A process for treating NOx in a diesel exhaust gas comprises absorbing the NOx in a solid absorbent and regenerating the absorbent by contacting it with CO-enriched gas.

[0001] The present invention relates to an exhaust gas treatment systemfor a diesel engine, and in particular to an exhaust system including aNOx-trap. Furthermore, the invention concerns a process forregenerating, i.e. removing absorbed NOx from, a NOx-trap in a dieselexhaust system.

[0002] Engine-out emission of carbon monoxide (CO), hydrocarbons (HC)and nitrogen oxides (NOx) depends on the air-to-fuel ratio (A/F),defined by equation (1):

A/F=mass of air consumed by the engine/mass of fuel consumed by theengine  (1).

[0003] The A/F where there is just enough air to complete the combustionof all hydrocarbons in the fuel is known as stoichiometry, and is at14.7 in gasoline engines. If the A/F is below this value, then theengine operates under excess fuel conditions, leading to incomplete fuelcombustion. The exhaust gas will then contain more reducing reactants(CO, HC) than oxidising reactants (O₂, NOx), and is called rich. If theA/F exceeds 14.7, then the engine operates under excess air conditions,giving rise to an exhaust gas that contains more oxidising reactantsthan reducing reactants, and the exhaust gas is called lean.

[0004] A common way of classifying the engine-out exhaust gascomposition is the lambda (λ) value, defined by equation (2):

λ=actual engine A/F/stoichiometric engine A/F  (2)

[0005] From equation (2) it will be seen that when the exhaust gascomposition is lean, λ≧1, and when the exhaust gas composition is rich,1≧λ.

[0006] In order to control NOx in exhaust gases from lean-burn gasolineengines, there has been devised a NOx absorber/catalyst which storesNOx, e.g. as nitrate, when an engine is running lean. In astoichiometric or rich environment, the nitrate is understood to bethermodynamically unstable, and the stored NOx is released and isreduced by the reducing species present in the exhaust gas. This' NOxabsorber/catalyst is commonly called a NOx-trap. By periodicallycontrolling the engine to run stoichiometrically or rich, stored NOx isreduced and the NOx-trap is regenerated.

[0007] A typical NOx-trap formulation includes a catalytic oxidationcomponent, such as platinum, a NOx-storage component, such as barium,and a reduction catalyst e.g. rhodium. One mechanism commonly given forNOx-storage during lean engine operation for this formulation is: (i)NO+½O₂→NO₂; and (ii) BaO+NO₂+½O₂→Ba(NO₃)₂. In the first step, the nitricoxide reacts with oxygen on active oxidation sites on the platinum toform NO₂. The second step involves adsorption of the NO₂ by the storagematerial in the form of an inorganic nitrate.

[0008] When the engine runs under rich conditions or at elevatedtemperatures, the nitrate species become thermodynamically unstable anddecompose, producing NO or NO₂ according to equation (iii) below. Underrich conditions, these nitrogen oxides are subsequently reduced bycarbon monoxide, hydrogen and hydrocarbons to N₂, which can take placeover the reduction catalyst. (iii) Ba(NO₃)₂→BaO+2NO+{fraction (3/2)}—O ₂or Ba(NO₃)₂→BaO+2NO₂+½O₂; and (iv) NO+CO→½N₂+CO₂ (and other reactions).In the reactions of (i)-(iv) above the reactive barium species is givenas the oxide. However, it is understood that in the presence of air mostof the barium is in the form of the carbonate or possibly the hydroxide.The above reaction schemes can be adapted accordingly for species ofbarium other than the oxide.

[0009] Using sophisticated engine management techniques and known fuelinjection components such as common rail, it is now becoming possible toadopt NOx-trap technology into the exhaust treatment systems for dieselengines. See, for example, EP-A-0758713 described below.

[0010] EP-A-0341832 (see also U.S. Pat. No. 4,902,487) describes aprocess for removing soot from diesel exhaust gas containing NOx bypassing the gas unfiltered over an oxidation catalyst to convert NO toNO₂, collecting the soot on a filter and using the NO₂-enriched gas tocombust the collected soot, the amount of NO converted to NO₂ beingsufficient to enable the combustion to proceed at a temperature lessthan 400° C.

[0011] The process described in EP-A-0758713 adopts the processdisclosed in EP-A-0341832 and further includes the step of removing NOxfrom the combustion outlet gas by means of a solid absorbent andregenerating the absorbent by intermittent contacting it with richexhaust gas composition.

[0012] It has been proposed to remove NOx from diesel exhaust gas byreacting it catalytically with injected ammonia. This process isgenerally called selective catalytic reduction (SCR) using ammonia. See,for example, WO-A-99/39809. Ammonia SCR does not necessarily require theexhaust gas composition to be made rich or equivalent, but it doesrequire the addition of a reductant to the exhaust gas.

[0013] We have now found that CO is effective to regenerate a NOxabsorbent in a diesel exhaust system. More particularly, we have foundthat the conditions for CO-promoted regeneration are approximately thesame as in ammonia SCR and a net-lean diesel exhaust gas composition ispreferred.

[0014] According to one aspect, the invention provides a process fortreating NOx in a diesel exhaust gas comprising absorbing the NOx in asolid absorbent and regenerating the absorbent by contacting it withCO-enriched gas.

[0015] According to a preferred embodiment, the CO-enriched gas isproduced by intermittently increasing the CO content of the exhaust gas,e.g. by injection into a conduit carrying it.

[0016] The gas contacting the absorber typically contains 1-20% v/v ofCO. This usually suffices to provide a 20-100-fold excess over the totalnumber of oxygen atoms present in the NOx leaving the absorbent duringregeneration. Suitably the gas is in the range 0.7 to 1.5 lambda,especially 1.0 to 1.2 lambda. It preferably contains at least enough,especially 1.5 to 3 times, the concentration of O₂ to oxidise all the COand other combustibles present. Intermittent increase of CO content neednot attain net-rich gas composition.

[0017] According to a further aspect, the invention provides an exhaustgas treatment system for a diesel engine comprising a NOx absorbercharged with solid absorbent for absorbing NOx from a diesel exhaust gasand means for introducing CO intermittently into the exhaust systemupstream of the absorber whereby the solid absorbent is regenerable bycontact with the CO-enriched gas.

[0018] According to a preferred embodiment, the exhaust system furthercomprises, upstream of the NOx absorber, a catalyst effective to promoteoxidation of at least NO to NO₂ and a filter effective to collect dieselsoot from the exhaust gas and hold it for combustion reaction with theNO₂ in the gas. In this embodiment, the present invention complementsour Continuously Regenerating Trap (CRT™) technology which is describedin our EP-A-0341832.

[0019] Preferably, the NOx absorbent comprises one or more of: (a)compounds of alkali metals, alkaline earth metals, rare earth metals andtransition metals capable of forming nitrates and/or nitrites ofadequate stability in absorbing conditions and of evolving nitrogenoxides and/or nitrogen in regenerating conditions; or (b) adsorptivematerials such as zeolites, carbons and high surface-area oxides, ormixtures of any two or more thereof.

[0020] Such a system preferably comprises a catalysed absorbent. By‘catalysed’ is meant that the absorbent is intimately associated withcatalytic material effective for the reaction of CO with NOx. Suchmaterial may be for example co-precipitated or co-impregnated orco-deposited with NOx absorbent or present as one or more sandwichedlayers or serial zones or as fine (e.g. 10-500 microns) particles on orin a layer of absorbent or among particles of absorbent. Whethercatalysed or not, the absorbent may be provided in one unit or asuccession of separate units. It is typically on a honeycomb substrate,such as a single honeycomb or multiple honeycombs.

[0021] Compounds (a) may be present (before NOx absorption) as compositeoxides, e.g. of alkaline earth metal and copper such as Ba—Cu—O orMnO₂—BaCuO₂, possibly with added Ce oxide, or Y—Ba—Cu—O and Y—Sr—Co—O.(For simplicity the oxides are referred to, but in situ hydroxides,carbonates and nitrates are present, depending on the temperature andgas composition). Whichever compounds are used, there may be presentalso one or more catalytic agents, such as precious metals, especiallyPGMs, effective to promote redox reactions between nitrogen oxides andCO.

[0022] The oxidation catalyst or the catalyst associated with theabsorbent or following it can be any that is active and stable.Typically these catalysts comprise one or more PGMs, especially Pt, Rh,Pd and combinations thereof, on a high-surface area washcoat on ahoneycomb structure. Detailed catalyst formulation is chosen accordingto which duty in the system the catalyst is to carry out. Suitablecatalysts have been described in the prior art and are available toskilled persons.

[0023] The catalysts and absorbent are suitably carried on a ceramic ormetal honeycomb substrate, the ceramic comprising one or more ofalumina, silica, titania, cordierite, ceria, zirconia, silicon carbideor other, generally oxidic, material. The honeycomb carries a washcoatand, in one or more layers thereon, the active catalytic and/orabsorptive material. The honeycomb has typically at least 50 cells persquare inch (cpsi), possibly more, e.g. up to 1000 cpsi, or up to 1200cpsi if composed structurally of metal. Generally the range 100-900 cpsiis preferred for the catalysts and absorbent.

[0024] Desirably, the filter is capable of trapping the soot withoutcausing excessive backpressure in the system and engine upstream. Ingeneral, ceramic, sintered metal or woven or non-woven wire filters areusable, and wall-flow honeycomb structures may be particularly suitable.The structural material of the filter is preferably porous ceramicoxide, silicon carbide or sintered metal. A coating such as aluminaand/or a catalyst such as La/Cs/V₂O₅ may be present. The soot isgenerally carbon and/or heavy hydrocarbons, and is converted to carbonoxides and H₂O. Certain embodiments of this principle are in commercialuse in Johnson Matthey's Continuously Regenerating Trap (CRT™)technology, and are described in above-mentioned EP-A-0341832 (see alsoU.S. Pat. No. 4,902,487), the teaching of which is incorporated hereinby reference.

[0025] According to a preferred embodiment, the system may furthercomprise, downstream of the absorber, a catalyst system effective topromote reactions of HC and CO with O₂ to H₂O and CO₂ and preferablywith NOx to N₂.

[0026] Advantageously, the system may further comprise sensors,indicators, computers and actuators, effective to maintain operationwithin desired conditions. Preferably a means for controlling COenrichment of the exhaust gas includes a computer which can be part ofthe engine management unit if desired. Control of the system can beregulated with open or closed feedback using information gathered fromthe sensors, indicators, etc. as explained below.

[0027] For regeneration of the NOx absorber the CO may be fed in assuch, subject to effective precautions against leakage, or as one ormore compounds decomposable to CO in the conditions of the system, forexample formic acid. Compounds introducing reductant, for example formicesters such as methyl formate, may be used. If reductant such as dieselfuel or HC derived therefrom is present, its concentration by carbonatoms is less than the CO, especially less than 10% of the CO. The CO isintroduced preferably by engine inlet adjustment.

[0028] Preferably the means for controlling the regeneration of theabsorber performs one or more of the following illustrative techniques:

[0029] (a) injection responsive to ultimate detection of NOx leakagefrom or “slip” past the NOx absorber;

[0030] (b) injection responsive to prediction based on input of data ondeliberate or load responsive engine management variation; and

[0031] (c) allowance for gas composition variations, for examplenon-steady conditions such as incomplete warm-up or weather. Usually theregeneration phase can be a small fraction of engine running time, e.g.0.1% to 5%, depending of course on operating conditions.

[0032] The required CO-rich gas is obtained, for example, by pilotinjection technique.

[0033] The control means may include sensors for at least one of: fuelcomposition; air/fuel ratio; exhaust gas composition (includingtail-pipe NO₂) and temperature at one or more points along the exhaustsystem; and pressure drop, especially over the filter. It may includealso indicator means informing the engine operator, computer meanseffective to evaluate the data from the sensor(s), and control linkageseffective to adjust the engine to desired operating conditions takingaccount of e.g. start-up, varying load and chance fluctuations.

[0034] In addition, the system may include routine expedients, forexample exhaust gas recirculation (E.G.R); and means such as cooling, orelectric heating, to adjust the temperature of the gas to a levelpreferred for nearer optimum operation of downstream components.

[0035] According to a further aspect, there is provided a diesel enginehaving an exhaust system according to the invention. The engine ispreferably of the direct injection common-rail type, especially usinginjection pressures in the range 1000-2000 bar, and advantageously isturbo-charged.

[0036] The engine may be the motive power for a vehicle, or may be astationary power source or auxiliary power source. It may be for a‘heavy duty’ vehicle, e.g. at least 3500 Kg (as defined by the relevantEuropean, US Federal or Californian legislation), or a ‘light duty’vehicle, including in particular a passenger car or light van and likelyto be operated according to the ‘urban cycle’.

[0037] Desirably the engine is fuelled with low-sulphur fuel, i.e.having less than 50 ppm, especially less than 10 ppm, by weight aselemental S. For operation with higher sulphur fuels, a SOx absorbentmay be used.

[0038] Most preferably, the engine according to the invention isoperated in compliance with the European IV standard.

[0039] The system may be structured within a single housing (“can”), orin separated housings according to engine design and under-floor orother space considerations. Thus for example for V-engineconfigurations, some or all of the elements of the system may bedisposed in parallel.

[0040] In order that the invention may be more fully understood,reference is made, by way of illustration only, to the Example below andthe accompanying drawing, which shows schematically a diesel engineequipped with a preferred embodiment of an exhaust system according tothe invention. In the drawing, full lines represent flow of gas anddotted lines represent flow of information or control power.

[0041]FIG. 1 illustrates an exhaust system 8 for a diesel engine 10,which engine having a common rail fuel feed 12 including valves 14 andhigh pressure pump 16 supplying diesel fuel, for example of under 50 ppmsulphur content. Exhaust system 8 comprises fuel feed 12 under thecontrol of computer 18 which computer is responsive inter alia to gascomposition at the outlet of catalyst 30 (described below) and topressure-drop across filter 26 (also described below), and is programmedto actuate valves 14 to inject fuel at the normal inlet stroke, and alsoto vary inlet conditions to produce CO-enriched exhaust intermittently.Engine exhaust gas passes via pipe 20 to can 22, at the inlet end ofwhich is catalyst 24, a low temperature light-off oxidation catalystsupported on a 400 cells/in² ceramic honeycomb. Catalyst 24 is designedto convert at least 70% of the NO in the normal gas to NO₂.

[0042] The gas leaving catalyst 24 passes into soot filter 26, which isof the ceramic wall flow type, and collects soot particles over 50 m.The NO₂ and surplus oxygen in the gas oxidise the soot at temperaturesaround 250° C. with no tendency to blocking.

[0043] Gas leaving filter 26 then enters NOx absorber 28 which includesalso PGM catalytic material. During normal lean operation of the engineand without CO enrichment by 18, absorber 28 absorbs NOx from theexhaust gas while it has capacity so to do. When, however, gas enrichedwith CO reaches it, the NOx is released and converted at least partly toN₂, for example by action of a reducing catalyst, such as rhodium. Thegas, now still containing CO, O₂ and possibly some NOx, passes intocatalyst 30, where these reactants are brought substantially to chemicalequilibrium comprising less environmentally harmful gases.

[0044] The process and system of the invention is expected to be capableof meeting European Stage 1V emission legislation.

EXAMPLE

[0045] A NOx absorber comprising a 400 cpsi monolith having wallthickness of 6/1000 of an inch, measuring 1 inch in length by 3 inchesin diameter and carrying a coating containing 62.8% w/w alumina, 23.8%w/w ceria-zirconia mixed oxide, 9.9% w/w magnesia, 1.7% w/w platinum,1.67% w/w palladium, 0.167% w/w rhodium, was subjected for 520 secondsat 200° C. to a synthetic gas stream to imitate the exhaust of a dieselengine, but containing NOx at 500 ppm. It was then fully regenerated byswitching the gas feed to net-rich for 180 seconds [‘rich ramp’] andoperated in this cycle at the same temperature and flow rate: 5 secondsregeneration phase in CO-enriched gas; 60 seconds absorption phase inuntreated lean gas.

[0046] The gas compositions at the absorber inlet and the NOxconcentrations at relevant points are shown in Tables 1 and 2respectively. TABLE 1 Rich ramp lean storage lean regeneration Lambda0.96 1.39 1.20 H₂O 10 10 10 CO₂ 10 10 10 O₂ 0.5 6.2 6.2 CO 2.0 0.04 5 H₂0.67 0.01 0.01 C₃H₆ 100 ppm 100 ppm 100 ppm NOx 500 ppm 500 ppm 500 ppm

[0047] TABLE 2 NOx ppm at absorber outlet Rich-regenerated: start 1 end300 Cycle 1: start 150 end 320 Cycle 2: start 100 end 360 Cycle 3: start100 end 390

[0048] It is evident that CO is capable of substantial regeneration ofthe absorber, and has the potential, after optimisation, forcomparability with rich regeneration.

1. A process for treating NOx in a diesel exhaust gas comprisingabsorbing the NOx in a solid absorbent and regenerating the absorbent bycontacting it with CO-enriched gas.
 2. A process according to claim 1,wherein the CO-enriched gas is produced by intermittently increasing theCO content of the exhaust gas.
 3. A process according to claim 2,wherein the exhaust gas includes CO, NOx, soot, HC and O₂, which processcomprising catalysing the oxidation of the NO to NO₂, collecting thesoot on a filter, combusting said soot by reaction with said NO₂,absorbing NOx present in gas leaving the filter in a solid regenerableabsorbent and regenerating the absorbent by intermittently increasingthe CO content of the exhaust gas and contacting said absorbent with aCO-enriched exhaust gas.
 4. A process according to claim 1, 2 or 3,wherein the CO content of the regenerating gas is in excess (by carbonatoms) over all other carbon-containing reductants present.
 5. A processaccording to claim 1, 2, 3 or 4, wherein the CO-enriched gas contains COin a 20-100 fold excess over the total number of oxygen atoms present inthe NOx leaving the absorbent during regeneration.
 6. A processaccording to any preceding claim wherein the CO-enriched gas is in therange 0.7 to 1.5, especially 1.0 to 1.2, lambda.
 7. A process accordingto any preceding claim, wherein the CO-enriched gas contains at leastenough O₂ to oxidise all the CO and other combustibles present.
 8. Aprocess according to any preceding claim wherein the gas is the productof combustion of a fuel containing less than 50 ppm w/w of sulphur. 9.An exhaust gas treatment system for a diesel engine comprising a NOxabsorber charged with solid absorbent for absorbing NOx from a dieselexhaust gas and means for introducing CO intermittently into the exhaustsystem upstream of said absorber whereby the solid absorbent isregenerable by contact with the CO-enriched gas.
 10. An exhaust systemaccording to claim 9, further comprising, upstream of the NOx absorber,a catalyst effective to promote oxidation of at least NO to NO₂ and afilter effective to collect diesel soot from the exhaust gas and hold itfor combustion reaction with the NO₂ in the gas.
 11. A system accordingto claim 9 or 10, wherein the NOx absorbent comprises one or more of:(a) compounds of alkali metals, alkaline earth metals, rare earth metalsand transition metals capable of forming nitrates and/or nitrites ofadequate stability in absorbing conditions and of evolving nitrogenoxides and/or nitrogen in regenerating conditions; or (b) adsorptivematerials such as zeolites, carbons and high surface-area oxides, ormixtures of any two or more thereof.
 12. A system according to claim 9,10 or 11, wherein the absorbent is catalysed.
 13. A system according toclaim 9, 10, 11 or 12, further comprising, downstream of the absorber, acatalyst system effective to promote reactions of HC and CO with O₂ toH₂O and CO₂ and preferably also with NOx to N₂.
 14. A system accordingto any of claims 9 to 13, including sensors, indicators, computers andactuators, effective to maintain operation within desired conditions.15. A diesel engine having an exhaust system according to any of claims9 to
 14. 16. An engine according to claim 15 which is of the directinjection common-rail type, optionally turbo-charged.
 17. An engineaccording to claim 15 or 16 operated in compliance with the European IVstandard.